The photonic band gap structures (PBG) are periodic structures that prohibit wave propagation for certain frequency bandwidths. For several years, research and studies have been conducted to use these structures in frequency ranges such as those used on microwave devices.
A method for realizing a structure of this type was proposed by the applicant, particularly in the French patent application no. 02 12656 of 11 Oct. 2002 and in the article entitled “Harmonic-less Annular Slot Antenna (ASA) using a novel PBG structure for slot-line printed device” IEEE AP-S 2003. These documents thus describe a method for realizing a PBG structure on a microwave device of the slot-line type realized on a metallized substrate, together with antennas of the annular slot type or Vivaldi type antennas using such structures to perform a filtering or a frequency adaptation of the said antenna.
As shown in FIGS. 1A and 1B, such a microwave device comprises a substrate 1 of which one face 2 has been metallized. A slot line 3 is realized by engraving the metal layer.
As shown in FIGS. 1A and 1B, the substrate 1 has a height h and is realized in a known dielectric material such as the materials known under the denomination “Ro4003” or “FR4”, the metal layer being realized preferably in copper or in any other conductive material.
In this case, the PBG structure is obtained by producing the patterns 4, namely patches, on the face of the substrate 1 opposite the face carrying the metal layer 2. The patterns or patches 4 are generally realized by engraving a metal layer and are found opposite the slot-line 3.
In a known manner, to obtain a photonic band gap structure, the patterns 4 repeat periodically and are spaced at a distance that gives the pattern repetition period. This distance sets the central frequency of the band gap when the patterns are identical. Hence, the distance is in the order of kλg/2 where λg is the guided wavelength in the slot-line 3 at the central frequency of the photonic band gap and k is a positive integer greater than or equal to 1.
The pattern 4 can be of any shape. However, the equivalent surface of the pattern determines the width and/or depth of the band gap.
To implement the filtering phenomenon of such a device, a device of the type of the one shown in FIG. 1A in which the substrate is constituted by the “Rogers Ro4003” with relative permittivity ∈r=3.38 and the metallizations are of copper of thickness 17.5 μm is simulated. In this case, the photonic band gap structure is composed of twelve metal discs 4 periodically spaced at a distance a=12.7 mm corresponding to the creation of a band gap centred at Fc(BI)=8.3 GHz, and the discs 4 have a radius r such that the ratio r/a=0.25.
As shown in FIG. 2, which gives the transmission S12 and reflection S11 coefficients according to the frequency, a band gap is obtained having a width of 900 MHz and centred on 8.25 GHz. In this case, the rejection of the central frequency 8.25 GHz is −17 dB.